Evolution of immune defence responses as incremental layers among Metazoa

Recent evidence show that the classical distinction between innate and acquired immunity can no longer be employed to thoroughly characterize immune defenses. As an example, both insects and vertebrates possess non-canonical defense activities that differ from those identified so far; namely, insects display immune memory and mammals show innate-like lymphocytes. At the base of these observations, it can be speculated that multicellular eukaryotes share immune cells deriving from ancestral granular and non-granular immunocytes as well as sets of soluble effectors and signaling immune-related molecules. On one hand, evolutionary pressure has selected genes coding for group-specific defense molecules; on the other, some cell types and gene products can be presently considered as being enriched with group-specific adaptations deriving from most ancient protostomes and deuterostomes. The immune defenses of more recent extant animal groups could then be seen as overlying layers constituted by both ancient cells retaining broad features for non-self defenses and more specialized cells and immunomodulatory molecules for group-specific capabilities.